1. Field of the Invention
The present invention relates to a liquid crystalline composition extremely useful in preparation of a retardation film and the like and, particularly, to a liquid crystalline composition which exhibits a biaxial liquid crystal phase. Further, the present invention relates to a retardation film having an optically anisotropic layer formed from the liquid crystalline composition and an elliptically polarizing film.
2. Background Art
An optical biaxial film having controlled refractive indices in three axial directions is useful in an optical art field in which polarization is utilized. Particularly, in the field of a liquid crystal display art, since polarization can be finely controlled with the biaxial film, an importance of such film is high. When such optical biaxial film as described above is prepared, it is an ordinary method that the biaxial film is prepared by biaxially drawing a film obtained from a polymer (for example, see JP-A No. 2-264905). When the biaxial film is prepared by the biaxial drawing, since refractive indices in three axial directions can be controlled by a draw ratio, they can comparatively easily be controlled to be of desired refractive indices.
Since the biaxial film using a biaxial liquid crystal has a merit such that a film thickness thereof can be extremely thin, compared with a biaxially drawn film which has been used in many cases, it is a useful measure for aiming for reducing thickness of a layer, weight or the like of a device to use the biaxial liquid crystal in the biaxial film. Further, a film prepared by the drawing has a problem in that it is inferior in dimensional stability and an optical property thereof is liable to be changed by moisture, heat and the like. By using a polymerizable biaxial liquid crystal, such problems as described above may possibly be solved.
However, when the biaxial film is prepared by using the biaxial liquid crystal, it is a problem that refractive indices in three axial directions can not arbitrarily be controlled. This is because the refractive indices in three directions of the biaxial film using a biaxial liquid crystal are determined nearly uniquely by the refractive indices in three directions of a compound (biaxial liquid crystalline compound) which exhibits a biaxial liquid crystal phase. Namely, in order to allow the refractive indices in three directions of the biaxial film to be desired refractive indices, there have not been other measures than synthesizing a biaxial liquid crystalline compound having the desired refractive indices. However, since the number of biaxial liquid crystalline compounds is small compared with a compound which exhibits a uniaxial liquid crystal phase (uniaxial liquid crystalline compound), it was extremely difficult to arbitrarily control the refractive indices in three directions.
In order to avoid such problems as described above, a proposal in which a biaxial liquid crystal phase is exhibited by mixing a rod-like liquid crystal and a discotic liquid crystal are mixed with each other has been made (for example, see Y. Rabin, Mol. Cry. Liq. Cry., Vol. 89, p. 67 (1982)). In such method as described above, since the refractive indices in three directions can be controlled by changing a mixing ratio of the rod-like liquid crystal to the discotic liquid crystal, the method is extremely convenient. At the proposal, many trials for exhibiting the biaxial liquid crystal phase by mixing the rod-like liquid crystal and the discotic liquid crystal have been performed (for example, seer R. Pratibaand N. V. Madhususana, Mol. Cry. Liq. Cry., Vol. 1, p. 111 (1985)). However, since the compatibility of the rod-like liquid crystal and the discotic liquid crystal was not favorable, there were problems in that, in the mixing of the two liquid crystal, crystallinity was disappeared or two types of liquid crystal phases caused a phase separation. Accordingly, in such mixing system as described above, the biaxial liquid crystal phase was not allowed to be exhibited.